Cryotron switching device



Sept. 24, 1963 c. R. sMALLMAN 3,105,156

cRYoTRoN swITcHING DEVICE Filed Feb. 4. 1957 United States Patent O3,165,156 CRYTRN Swirl-GENS DEViCE Carl Russell Smallman, Lexington,Mass., assigner to Arthur D. Little, Inc., Cambridge, Mass., acorporatien ci Massachusetts Filed Feb. 4, i957, Ser. No. 637,945 l2Claims. (Cl. 307-385) This invention relates to a device for switchingor controlling electrical current and more particularly to a circuitcomponent utilizing a body of superconductive material.

Various superconductive materials are known which are capable of achange of state from one of iinite electrical resistance to one of zeroresistance. For example, a body of lead cooled to 7.2 degrees Kelvinsuddenly drops to zero resistance. The temperature at whichsuperconductive materials undergo such transition is dependent on themagnetic field about the material. The critical temperature of 7.2" K.for lead supposes a zero magnetic iield. As the iield increases towardapproximately 800 oersteds the transition temperature drops toward zero,and at intermediate temperatures there is a ield which, if exceeded,will cause the lead body to change from superconducting state to a stateof iinite resistance. Thus for any given temperature below criticaltemperature there is a predetermined value of magnetic field above whichlead undergoes transition from the superconducting state, and thetransition between superconduction and linite resistance, can beeiiected by varying the magnetic iield respectively below and above thepredetermined value. Above the critical temperature no reduction ofiield can restore superconduction. Herein the term superconductive isused to designate the capability of the body to change between theabove-mentioned states, while superconducting or superconductiondesignates the zero resistance state.

The possibility of transferring a superconductive -body betweensuperconducting and resistance states by varying the iield around thebody above and below a predetermined value is utilized in electricalcircuit element as follows: Around a short body of superconductive wire,such as tantalum (Ta) is wound a coil of wire, such as niobium. Theelement so formed may be held below the critical temperature of tantalum(4.4 K.) by immersion in liquid helium at 4.2 K. In the absence of amagnetic held, a primary current applied to the tantalum body willencounter Zero resistance in iiowing through the superconductingtantalum body. If a control current is applied to the niobium coilsuliicient to produce a magnetic eld of approximately l() oersteds, thetantalum body will change partially or wholly to resistance state andpresent a nite resistance to the primary current owing therethrough. Inchanging to resistive state the tantalum body acts as a Valve or switchcomponent. For example, it the primary current has an alternate zeroresistance path, the finite resistance, although small, will in eiectswitch the primary current to the alternate path.

In one useful example, when two like components are connected inparallel with a current source, and the control coil of one body isconnected in series with the other, and vice versa, the two componentsform a flip-flop circuit wherein the tendency of one body to change toresistive state, is reinforced by the tendency of the other body toremain superconducting. However, such a circuit requires in addition,signal input means for influencing the states of the respectiveilip-ilop components, and usually also signal output means for detectingthe state of the respective bodies.

One object of the present invention is to provide a superconductivecircuit component which simplies the f, anales Patented Sept. 24, 1963means by which input signals or output signals are applied or detected.

According to the inventiona circuit component comprises asuperconductive body responsive to a predetermined eld to change betweena state of superconduction and a state of iinite resistance, controlmeans for applying a magnetic iield to said body thereby to influencesaid change of state, and superconductive means disposed in the iield ofsaid control means.

While a control coil has been previously mentioned other forms ofmagnetic control means are useful in the production of a control field.The control means need not supply the whole ield applied to the body solong as it influences the transition between states. The currentconductor may be superconducting means in the iield of the controlmeans, or may produce its own iield common to the body or control means,as will be apparent from the illustrative embodiments shown in theaccompanying drawings in which:

FIG. 1 is a plot of transition temperature against magnetic iieldapplied to various superconducting bodies;

FIG. 2 is a similar plot illustrating a predetermined iield value atwhich a superconductive body changes between superconducting andresistive state;

FIG. 3 is a schematic diagram of a llip-flop circuit; and

FIG. 4 is an isometric view of a circuit component shown schematicallyin FIG. 3.

As `shown in FIG. 1 various elements are capable of superconduction,depending upon the temperature and magnetic iield of their environment.In this ligure are shown the transition curves of aluminum (Al),thallium (Tl), indium (In), tin (Sn), mercury (Hg), tantalum (Ta),vanadium (V), lead (Pb) and niobium (Nb). For each of these elements thecurve is a plot ofthe transition temperature as a function of theapplied magnetic eld. Below the curve the element is superconducting,and above the curve the element has a iinite resistance usually lessthan the resist-ance at room temperature.

As shown in FIG. 2 the transition curve is the boundary between thesuperconducting region and finite resistance Y region of a givenelement. For a given temperature environment T there is a predeterminedmagnetic tield value H `at the transition point or zone. Increasing theiield above the predetermined value H' destroys superconduction, whilereducing the tield below the predetermined Value establishessuperconduction.

The flip-flop or bistable circuit shown in FIG. 3 forms two paths forcurrent ilowing from a source I to a current collection point which forsimplicity is shown as a ground return to the current source I. One pathfrom the source I may be traced through a connection 1, asuperconducting body G13', thence through a connection 7 and a controlwinding C45 to the ground return. Similarly the second path is tracedthrough parts 2, G24, S, C35 and 9 to ground.

Each of the bodies G13 and G24 comprises a gate of superconductivemetal, such as tantalum, held below its critical temperature byimmersion in liquid helium. Gate G13 and coil C46 are connected inseries such that current through gate G13 Hows through the control coilC46 of gate G24; and conversely gate G24 and coil C35 being in series,current through gate G24 iiows through control coil 35 of gate G13.

Except under theoretical conditions, the current through the gates G13and G24 will tend to be more tand more unbalanced until one of the gatesconducts all the current. For example, if superconducting gate G13carries more current, that greater current flowing also through controlcoil C46 will xapply la magnetic eld which drives gate G24 towardresistive state with a regenerative action. When Kgate G24 reachesresistive state, current from the Y source yI, having the choice of aZero resistance path through GES and a linite resistance path through`gate G24 will ilow entirely through gate G13 and control coil C46,thereby establishing a magnetic field which stably holds the `gate G24in resistive state until circuit conditions are changed.

The stably unbalanced condition of vthe flip-flop circuit may bereversed by supplying a current signal to the input or set terminals slor s2 of control coils C1 and C2 wound around gates GILS and G24respectively. Each of the input terminals sl and s2 may be connectedthrough switching means S to a current source i. While the switchingmeans is shown schematically as a simple contactor, Various other meansmay beused to `apply a signal to the set terminals.

Assuming gate G13 to be superconducting, if a current signal is appliedto set terminals sl and coil C1, superconduction in gate G13 will bedestroyed by the resulting magnetic field and its finite resistance willappear in the previously non-resistive path therethrough. The abovedescribed iregenerative eiect will then tend to restore superconductionin gate G24 until the circuit is stably unbalanced with all currentiiowing through the gate G24. 'llhe signal at terminals s1 may then beremoved without changing the unbalance. Similarly -a signal at terminalss2 will establish current through the opposite gate G13.

The alternative conditions of unbalance, are detected by output gates Gand G6 embraced within coils C35 and C45 respectively. lf current isestablished through either of gates Gl3 or G24, it passes through one orthe other of the control coils C35 and C46 producing `a magnetic fieldwithin Ithe coil as described above. Accordingly `one of thecorresponding output gates G5 or G6 will be caused to have la finiteresistance. For example, if `gate G13 is in resistive state, the outputgate G5 embraced by t-he field of transfer coil C35 will also be held inresistive state. A current source -I connected in parallel with a meterM and through terminals t1 and the output gate G5 will cause the meterto indicate the resistive state by a given deflection. With la likedetecting circuit connected to the output terminals t2 of output gateG6, the state of -unbalance of the flip-flop circuit may be determinedwith reference to the meters M, only one of which will indicateresistance at a time.

As shown in FIG. 4, each y:side of the bi-stabl-e circuit of PEG. 3 `maybe formed as a single component D. By way of example, the left handcomponent may comprise two gate wires G5 and G13 each approximately0.009 inch in diameter and one half inch long. Preferably the gate wireslare tantalum lhaving -a :critical temperature (i.e. maximumsuperconducting temperature in zero magnetic iield) of about 4.4 K., 0.2K. above the boiling point of liquid helium. The individual wires yarecoated with a thin (0.0005) layer of suitable insulation such as enamel,`and have leads or terminals 1, 7, and t1, preferably niobium, connectedto their ends by welds w. Wound around the two `gates are lapproximately150 parallel turns each of two wires C1 land C35. The wires arepreferably niobium, 0.003 inch in diameter, coated with a suitableinsulation.

Tantalum gates are niobiurn coils and lead connections are describedsince tantalum is superconducting just below the boiling point of liquidhelium and can be changed to resistive state by `a small magnetic field.On the other thand niobiums `transition curve lies wholly outside thesuperconducting region of tantalum, and hence the niobium coils yandlead connections will have zero resistance under field values adjacentthe transition curve of tantalum. From this example it will be apparentthat various elements are suitable las superconductive gates and that,if desired, other elements having higher transition curves may beused ascoils and connections.

While both input and control coils vare shown wound on the regenerativeand output gates, it should be understood that the input signal may beapplied other than by a coil so Wound, while still retaining theadvantage of embracing both the regenerative and output gates within thesame control field. it two controls are used they need not be woundparallel, but may be wound separately or may be conductors in other thancoiled form. Further, it should be understood that more than two coilsmay be used as inputs or controls for each component D, and that morethan two gate bodies may be embraced within the field of the coils. Thusthe example yof FIGS. 3 and 4 embraces various combinations andequivalents within the scope of the appended claims.

I claim:

l. An electrical circuit component comprising a superconductive bodyresponsive to -a predetermined magnetic iield to change between a stateof superconduction and a state of finite resistance, control means forapplying a magnetic field to said body thereby toV inline ce said changeof state, and superconductive means disposed in the field of saidcontrol means and influenced to change state concomitantly with saidbody, said control means being disposed such that th field atiectingsaidbody and said superconductive means is substantially continuous.

2. An electrical circuit component comprising a superconductive bodyresponsive to a predetermined magnetic lield to change between `a stateof superconduction and a state or" finite resistance, control meansdisposed to apply a 'magnetic field to said body to efi'ect said changeof state, and ia second superconduetive body disposed in the iield ofsaid control means yfor indicating the state of the nrst said gate body,said control means being disposed such that the field affecting therespective bodies is substantially continuous.

3. An electrical circuit component comprising a plurality ofsuperconductive bodies responsive to a predetermined magnetic eld tochange between a state of superconduction and a state of finiteresistance, and control tmeans for applying a magnetic field to saidbodies, one of said bodies comprising a gate and another of said bodiescomprising output means, land said control means comprising input meansapplying a magnetic eld to both said gate and output means, said controlbeing disposed such that the iield affecting the respective bodies issubstantially continuous.

4. An electrical circuit component `comprising a plurality ofsuperconductive bodies responsive to a predetermined magnetic iield toychange between a state of superconduction and a state of iiniteresistance, and a plurality of control means respectively for applying`a magnetic iield to said bodies, one of said bodies comprising a gateand vanother of said bodies comprising output means, 'and one of saidcontrol means comprising input means applying a magnetic field to bothsaid gate yand output means, said control means being disposed such thatthe field aifecting the respective bodies is substantially continuous.

5. A Hip-flop comprising two units, each unit including at least twosuperconductive bodies responsive to a predetermined magnetic field ltochange between a state of supenconduction and `a state of iiniteresistance, and each unit including control means for Iapplying amagnetic iield to both bodies of a unit collectively, current supplymeans connected to one of the bodies of each unit, one body of each unitbeing connected to the control means of the other unit, currentcollecting means connected to the control means of each unit, each ofthe other bodies of each unit having out-put terminals, and input meansfor establishing resistance to current through said one body of one unitLand the control means of the other unit, thereby to permit both bodiesof said other unit to conduct unresisted current so that no resistanceis Ioffered to current between the output terminals of said .other unit,the control aneans of each unit being disposed such that the lieldaffecting its respective bodies is substantially continuous.

6. A flip-iiop comprising two units, each unit includ-V ing 'at leasttwo superconductive bodies responsive to a predetermined magnetic fieldto change between a state of superconduction and ya state of niteresistance, and each unit including at least two control meansrespectively for 'applying a magnetic i'leld to both bodies `of a unitcollectively, current supply means connected to one of the bodies ofeach unit, one body vof each unit being con nected to one of the controlmeans of the other unit, current collecting means `connected to -saidone control means of each unit, each of the other control means of `eachunit having independent input terminals, and each of the other bodies ofeach unit having output terminals, whereby a `current `applied to theinput terminals of one unit destroys the superconduction of that unitand establishes resistance to current through the control means of theother unit, thereby to permit both bodies of said other unit to conductunresisted current so that no resistance is offered to `current betweenthe output tenminals of said other unit, the control means of each unitbeing disposed such that the field yaffecting its respective bodies issubstantially continuous.

7. An electrical circuit component adapted to fonm with a like componenta bistable circuit #having an input, a transfer and an output func-tion,said component comprising an output gate and a transfer gate, each gatecom- @rising a superconductive body responsive to a predeterminedmagnetic field to change between a state of superconduction and a stateof finite resistance, and a transfer control and an input control, eachof said controls comprising a current conductor disposed to apply amagnetic field to both of said gates thereby to influence `a change ofstate in said gates simultaneously, said controls being disposed suchthat the portions of their fields Iaffecting the gates are substantiallycontinuous.

8. An `electrical circuit comprising, two components each including anoutput gate and -a transfer gate, each gate comprising a superconductivebody responsive to a predetermined magnetic eld to change between astate of snperconduction and 1a state of fini-te resistance, and atransfer control and an input control, each of said controls comprisinga current conductor disposed to yapply a magnetic field to both of saidgates thereby to influence a change of state in said gatessimultaneously, the transfer gate of each component being connected in aseries With the transfer control of the other component, and both ofsaid series being connected in parallel with each other, and commoncurrent-supply and current-collection connected to said series, wherebya current supplied to the nput control of one component ycauses thetransfer gate of said one component to change to a state of iiniteresistance and divert supply 4current through the other unit to thetransfer control of said one unit thereby to lhold both the transfer andoutput gates of said one uni-t in a state of iinite resistance, thecontrols of each component being disposed with Irespect to theirrespective gates such that the portions of their elds affecting theirrespective gates are substantially continuous.

9. An electrical gating element comprising two conductors arrangedside-by-side, the conductors having critical temperatures and magneticfield transition characteristics between resistive and superconductivestates, means for maintaining said conductors at a temperature such thatthey Iare superconductive in the absence of a magnetic field, and acontrol coil surrounding and wound around both or the `conductors toestablish a Imagnetic field to destroy superconductivity in bothconductors.

1i). A gating element as deiined in claim 9 in Iwhich the control coilis itselt` of superconducting material.

1l. A bistable circuit comprising two transfer conductors and twosensing conductors, all of a material having critical temperature andmagnetic field chlaracteristics between superconductive `and resistivestates, a control coil surrounding one of the transfer conductors andone of the sensing conductors, a second control coil surrounding theother transfer and sensing conductors, means for connecting each4transfer conductor with the control coil surrounding the other transferconductor, and means for selectively establishing current flow in one ofthe transfer conductors whereby said current will establish a magneticiield to destroy superconductivity in the other transfer conductor andits associated sensing conductor.

12. A bistable circuit as defined in claim l1 in which the control coilsare of a material Ito maintain superconductivity in the presence oftheir own magnetic fields.

References Cited in the le of this` patent UNTED STATES PATENTS1,825,855 Craig ont. 6, 1931 1,948,209 Fichandler Feb. 20, 19342,666,884 Ericsson et al. Ian. 19, 19574 2,682,615 Sziklai June 29, 19542,832,897 Buck Apr. 29, 1958 2,852,732 Weiss Sept. 16, 1958

1. AN ELECTRICAL CIRCUIT COMPONENT COMPRISING A SUPERCONDUCTIVE BODYRESPONSIVE TO A PREDETERMINED MAGNETIC FIELD TO CHANGE BETWEEN A STATEOF SUPERCONDUCTION AND A STATE OF FINITE RESISTANCE, CONTROL MEANS FORAPPLYING A MAGNETIC FIELD TO SAID BODY THEREBY TO INFLUENCE SAID CHANGEOF STATE, AND SUPERCONDUCTIVE MEANS DISPOSED IN THE FIELD OF SAIDCONTROL MEANS AND INFLUENCED TO CHANGE STATE CONCOMITANTLY WITH SAIDBODY, SAID CONTROL MEANS BEING DISPOSED SUCH THAT THE FIELD AFFECTINGSAID BODY AND SAID SUPERCONDUCTIVE MEANS IS SUBSTANTIALLY CONTINUOUS.